The Schmidt Trigger circuit is well known as a switching circuit which operates in a hysteretic manner.
FIG. 1 shows an example of the Schmidt Trigger circuit. As is shown in FIG. 1, the base of an npn transistor Q1 is connected to an input terminal Vin. A resistor R1 has one end connected to the collector of the transistor Q1, and the other end connected to a power source terminal V.sub.DD. A resistor R2 has one end connected to the emitter of the transistor Q1 and the other end connected to an earth terminal Vss. The base of an npn transistor Q2 is connected to the collector of the transistor Q1, while the collector of the transistor Q2 is connected to an output terminal Vout. A resistor R3 has one end connected to the collector of the transistor Q2 and the other end connected to the power source terminal V.sub.DD. The emitter of the transistor Q2 is connected to the one end of the resistor R2.
The maximum hysteresis error of the above circuit is determined by the values of the resistors R1-R3. The output voltage of the circuit varies from a low voltage (hereinafter called "L") to a high voltage (hereinafter called "H") when the transistor Q1 is turned on, while it varies from "H" to "L" when the transistor Q2 is turned on. That is, when one of the transistors Q1 and Q2 is on, the other is off.
The Schmidt Trigger circuit shown in FIG. 1 operates in a hysteretic manner as shown in FIG. 2. Then, this operation will be explained with reference to FIGS. 1 and 2.
When the transistor Q2 is on, and the transistor Q1 is off, the output voltage is "L". Thereafter, the input voltage Vin increases, and reaches a value which satisfies the following formula (1). At this time, the transistor Q1 is turned on, and accordingly the transistor Q2 is turned off. Thus, the output voltage varies from "L" to "H". EQU Vin&gt;V.sub.th(L.fwdarw.H) =V.sub.DD .times.R2/(R2+R3)+V.sub.BEQ1 (1)
where V.sub.BEQ1 represents the threshold voltage of the transistor Q1, and V.sub.th(L.fwdarw.H) represents that threshold voltage of the Schmitt trigger circuit which is assumed when the output voltage varies from "L" to "H". Further, the earth voltage is applied to the earth terminal Vss, and the saturation voltage of the transistor Q2 is ignored.
Thus, the output voltage Vout varies in the order of a, b, c, d, and e in FIG. 2.
When the input voltage Vin reduces, and reaches a value which satisfies the following formula (2), the transistor Q2 is turned off, and the transistor Q1 is turned on. Thus, the output voltage varies from "H" to "L". EQU Vin&lt;V.sub.th(H.fwdarw.L) =V.sub.DD .times.R2/(R1+R2)+V.sub.BEQ1 (2)
where V.sub.BEQ1 represents the threshold voltage of the transistor Q1, and V.sub.th(H.fwdarw.L) represents that threshold voltage of the Schmitt trigger circuit which is assumed when the output voltage varies from "H" to "L". Further, the earth voltage is applied to the earth terminal Vss, and the saturation voltage of the transistor Q2 is ignored.
Thus, the output voltage Vout varies in the order of e, d, f, b, and a in FIG. 2.
In the above embodiment, the values of the resistors R1 and R3 are determined so as to satisfy the following formula (3): EQU Vth(L.fwdarw.H)&gt;Vth(H.fwdarw.L) R1&gt;R3 (3)
This means that the hysteresis characteristic is obtained by setting to different values the threshold voltages V.sub.th(L.fwdarw.H) assumed when the output voltage varies from "L" to "H", and V.sub.th(H.fwdarw.L) assumed when the output voltage varies from "H" to "L".
However, as is evident from the formula (1), the threshold voltage V.sub.th(L.fwdarw.H) assumed when the output voltage varies from "L" to "H" is determined by the resistance ratio between the resistors R1 and R3. Similarly, as is evident from the formula (2), the threshold voltage V.sub.th(H.fwdarw.L) assumed when the output voltage varies from "H" to "L" is determined by the resistance ratio between the resistors R1 and R2. Accordingly the threshold voltages V.sub.th(L.fwdarw.H) and V.sub.th(H.fwdarw.L) depend upon each other, and hence, must be set to values falling within a very narrow range.
In addition, in a case where the Schmidt Trigger circuit comprises an integrated circuit, great variations may exist among Schmidt Trigger circuits if the ratio of the largest resistance of the resistors R1-R3 to the smallest resistance of them is set to a large value. Further, as can be understood from the formulas (1) and (2), the threshold voltages V.sub.th(L.fwdarw.H) and V.sub.th(H.fwdarw.L) vary in accordance with a change in the power source voltage V.sub.DD. Hence, a constant voltage source dedicated for supplying the threshold voltages is required in the integrated circuit.
It is the object of the invention to provide a hysteresis circuit in which the threshold voltage V.sub.th(L.fwdarw.H) assumed when the output voltage varies from "L" to "H", and the threshold voltage V.sub.th(H.fwdarw.L) assumed when the output voltage varies "H" to "L" are set independently of each other, and do not vary in accordance with a change in the power source voltage of the circuit.